1. Field of the Invention
The present invention relates to papermaking and the use of high lignin content recycled fiber sources as furnish for bleached kraft paper and paperboard.
2. Description of Related Art
Bleached kraft paper and paperboard are utilized to package food and other sanitation sensitive products. In many cases, food, drugs and other products intended for human consumption or use in intimate contact with a human body are packaged in direct contact with respective paper or paperboard enclosures. Consequently, most manufacturers and producers of such paper and paperboard maintain food grade production facilities and procedures. Only food grade additives are combined with natural fiber furnish to lay such paperboard.
Concern for sanitation and chemical contamination in the papermaking process has, in the recent past, greatly restricted acceptable fiber sources to substantially virgin fiber and the waste of a very few, strictly controlled, paper converting plants. Such concern for biological and chemical contamination has contributed to a virtual abstention from using post-consumer waste paper as a fiber supply for bleached paper and paperboard.
However, biological and chemical contamination concerns are not the only prior art restraints from recycling post-consumer waste paper into bleached kraft paper and paperboard. The mass dominance of post-consumer paper collected for recycling is high yield material having less than half of the naturally present original lignin removed by respective digestion/delignification procedures. For example, newsprint, typically, is mechanically ground wood having no native lignin content removed. Typical corrugated container board is 75-80% yield material. Consequently, bleaching costs for old corrugated containers (OCC) are extremely high in comparison to virgin material processing thereby exacerbating the adverse environmental impact of fiber bleaching.
Additionally, post-consumer waste paper fiber is thoroughly laced with metal fasteners, plastic film coatings, adhesives, heavy metal dyes and ink pigments, all of which must be removed prior to any attempt to reform a new paper web from such fiber.
The high quality paper and paperboard products used for packaging, especially food packaging, are blade coated on a high-speed paper machine, operating in a range of at least, e.g. 800-1200 ft/min. Blade coating produces a superior finish, but the process is extremely sensitive to contaminants, especially plastics and stickies. Plastics and stickies present in the paperboard become lodged under the blade and produce streaks and scratches during coating. In addition, contaminants lodged under the blade cause coating to be splashed onto adjacent equipment causing scale buildup. The scale quickly builds to a level where small bits of dried coating break off and fall onto the paperboard below, becoming lodged in the finished reel. When this material reaches the customer's converting operation the result can be disastrous, with bits of coating deposited on offset blankets and gravure cylinders, causing poor print quality for the entire converting run.
Thus, blade coated paperboard having a significant level of contaminants, e.g. above 20 ppm, is essentially unsalable. For this reason, old corrugated containers (OCC) with high contaminant levels have been viewed as a highly unlikely source of recycle material for producing blade coated paperboard. The corrugated portion of such materials is often made from "reject" hardwood pulp, i.e. pulp too dirty to be used for other purposes.
The economics of paper production generally dictate against the use of secondary recycle fiber sources in the production of high quality bleached kraft paper and paperboard. However, economics alone no longer strictly control the production of paper, as consumers have become aware of the environmental impact of paper production and disposal, and have begun demanding recycled fiber content in the packaging for the products they purchase. It has therefore become imperative that methods be developed for incorporating recycled fiber in high quality paper products in spite of the cost and difficulties in doing so.
Several schemes are known in the art for recycling corrugated paperboard, focusing mainly on the cooking procedure. Thus, Japanese Published Application JP-A-57/167,475 discloses alkaline digestion at 130.degree.-170.degree. C., followed by blowing at a temperature of less than 100.degree. C., removal of foreign matter and bleaching. The bleached pulp which results can be used in printing grade paper. A related application is Japanese Published Application JP-A-57/16,990.
U.S. Pat. No. 5,147,503 is also concerned primarily with the digesting operation, disclosing digesting with an aqueous alkaline cooking liquor containing sodium sulfide at 160.degree.-180.degree. C. The '503 patent also discloses an initial pulping and cleaning process including "dry pulping" (20-30% solids) with some removal of contaminants, followed by dilution to 3-4% solids, screening and centrifuging. The aqueous pulp mix is then dewatered and digested. The final product in the '503 patent is described as pulp suitable for bleaching to produce white paper products.
These references do not disclose, for example, the production of high quality, blade coated paperboard containing recycled material.
It is therefore an object of the present invention to provide high quality, blade coated paper and paperboard containing recycled material, especially recycled corrugated paper.
It is also an object of the present invention to provide an economical process for recycling high lignin paper fiber into bleached kraft paper and paperboard.
Another object of the present invention is to teach a process for recycling high lignin paper fiber from post-consumer sources into sterile bleached kraft paper and paperboard.
Another object of the present invention is to teach a process for separating natural wood fiber from a miscellany of contaminants present in post-consumer waste paper.